The congestion can’t lead to a call drop :
case 1 : there is no congestion and the TCH is allocated for this one user–> call setup successful.
case 2 : there is congestion and the TCH is not allocated –> call setup failure.
As you can see, the congestion impacts the call setup, not the call drop. Call drop is occurring after the TCH has been assigned : the radio link is already allocated. If there is congestion in the cell, it will not release an “active” TCH. It will just prevent new TCH allocation.
Regarding your other question, i cannot answer (i’m not from ericsson)